puppeteer/website/versioned_docs/version-21.1.1/api/puppeteer.page.waitforfunction.md

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Page.waitForFunction

Page.waitForFunction() method

Waits for a function to finish evaluating in the page's context.

Signature:

class Page {
  waitForFunction<
    Params extends unknown[],
    Func extends EvaluateFunc<Params> = EvaluateFunc<Params>,
  >(
    pageFunction: Func | string,
    options?: FrameWaitForFunctionOptions,
    ...args: Params
  ): Promise<HandleFor<Awaited<ReturnType<Func>>>>;
}

Parameters

Parameter	Type	Description
pageFunction	Func | string	Function to be evaluated in browser context
options	FrameWaitForFunctionOptions	(Optional) Options for configuring waiting behavior.
args	Params

Returns:

Promise<HandleFor<Awaited<ReturnType<Func>>>>

Example 1

The Page.waitForFunction() can be used to observe viewport size change:

import puppeteer from 'puppeteer';
(async () => {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();
  const watchDog = page.waitForFunction('window.innerWidth < 100');
  await page.setViewport({width: 50, height: 50});
  await watchDog;
  await browser.close();
})();

Example 2

To pass arguments from node.js to the predicate of Page.waitForFunction() function:

const selector = '.foo';
await page.waitForFunction(
  selector => !!document.querySelector(selector),
  {},
  selector
);

Example 3

The predicate of Page.waitForFunction() can be asynchronous too:

const username = 'github-username';
await page.waitForFunction(
  async username => {
    const githubResponse = await fetch(
      `https://api.github.com/users/${username}`
    );
    const githubUser = await githubResponse.json();
    // show the avatar
    const img = document.createElement('img');
    img.src = githubUser.avatar_url;
    // wait 3 seconds
    await new Promise((resolve, reject) => setTimeout(resolve, 3000));
    img.remove();
  },
  {},
  username
);